MEMS pressure sensors are well known, and typically have a piezo-resistive or a capacitive read-out, to detect movement of a suspended structure when it is subjected to an external pressure. One example of known device uses a thin circular suspended SiN membrane, which is used within a capacitive MEMS pressure gauge.
The cavity underneath the SiN membrane is hermitically closed by means of a PECVD SiN process, for example using a 2 μm thick PECVD SiN film. The final device performance is largely determined by the physical, mechanical and structural properties of this film and the thickness necessary to close the sacrificial etch holes. The density and composition of this membrane determine the hermeticity, out-gassing behaviour, and internal stress. The stress together with the thickness determine the membrane rigidity and hence the sensitivity of the pressure sensor.
Critical systems in medical applications, alternative energy applications, smart buildings, engine control (e.g. gas and fuel inlet pressure), and automotive safety applications such as tyre monitoring systems, require pressure sensors that deliver accurate and predictable output over their lifetime in order to track changes in operation before those changes become critical. MEMS pressure sensors are now widely used in such applications.
With increasing miniaturization, MEMS pressure sensors also start to suffer performance issues. For instance, issues arise relating to insensitivity, inaccuracy and signal drift. Although state-of-the-art deposition tools and lithographic techniques are being employed, it is impossible to ensure that all MEMS devices are uniformly deposited or have identical geometries. Since no two MEMS devices are exactly the same, there has to be some means of calibrating them to cancel out those irregularities. As it is highly impractical to measure individual device parameters in an industrial fabrication environment (for example deflection profiles in relation to an externally applied force), it is important to develop methods for internal calibration to ensure that two MEMS devices function or measure in exactly the same way.
Self-calibration also is needed because the devices can be exposed to harsh environments or remain dormant for long periods. In some cases the device should wake up and recalibrate itself to account for changes resulting from temperature differences, changes in the gas or liquid ambient surroundings, or other conditions that might affect the properties.
Currently most MEMS pressure sensors use a hermetically sealed membrane that seals a reference cavity which is at a certain gauge pressure (in some cases the gauge pressure is a vacuum). The external pressure is measured because the pressure difference between the external pressure and the gauge pressure generates a force on the membrane, which causes the membrane to deflect. This deflection is then measured by piezoresistive, capacitive or optical sensors. There are several difficulties related to this conventional pressure sensor design.
For example, the gas pressure in the reference cavity needs to be very stable in order to avoid signal drift. This requires a very high level of hermeticity without out-gassing of the membrane.
However, in order to have a large deflection and optimum sensitivity, the thickness of the membrane should be thin (or it should have a large area). Since it is difficult to make very thin membranes hermetic, these are conflicting requirements and lead to a larger sensor size.
If the reference cavity is at a certain pressure, this pressure will be temperature dependent (according to Boyle's law). The sensor thus becomes temperature dependent. The sensitivity of the pressure sensor is determined by the amplitude of membrane deformation which is in turn defined by the thickness, diameter, and yield strength of the membrane. For a sensor with capacitive read-out, the sensitivity depends also on the distance of the plates becoming larger for higher pressures. The total dynamic range of deflection-based pressure sensors is also limited by the maximum deflection of the membrane.
It is clear therefore that any variation in membrane thickness, diameter, and stress has a significant impact on the resulting deflection profile of the membrane which affects not only the absolute capacitance reading but also accuracy and precision of the read-out. Due to process variations during fabrication, no two microstructures have the same geometric and material properties which will induce small variations in dimension, mass, stiffness that will significantly affect performance. For instance, a 10% variation in membrane thickness can cause a 50%-100% change in a microstructure's stiffness and pressure sensitivity if the membrane contains compressive and tensile layers. The actual dependence depends on the way the membrane is arranged to deflect. For instance, the thickness dependence for stress dominated membranes is proportional to the thickness whereas for bending stiffness dominated membranes it is proportional to thickness to the power of 3.